Various types of electronic consumer and office products which contain the capability to generate hard copy, such as lap top computers, facsimile machines and the like, may contain thermal printers which incorporate a thermal print head that in combination with thermally sensitive paper generates the desired hard copy images. Various types of thermal print heads have evolved with the proliferation of such equipment. There are several common types of stationary line printing or row of dots thermal print heads, typically described by the location on a substrate of thermal elements which effect the actual production of images, including: center, near edge, and true edge print heads.
A center type print head typically has resistive printing elements located at the center of a large planar surface of a substrate. The resistive elements may be disposed on a layer or strip of glaze which elevates the printing elements from the substrate somewhat enhancing contact with the thermally sensitive print medium which is moved across the print elements parallel to the large planar surface of the substrate
A near edge type print head typically has resistive printing elements located near an edge of a large planar surface of the substrate. Like the center type print head the thermally sensitive medium travels parallel to the large planar surface of the substrate.
FIGS. 1 and 2 illustrate center and near edge type thermal print heads according to the prior art. A substrate 10, typically alumina, provides a base for a series of layers which are laminated thereon. A bead of glass 12 is disposed on the substrate 10 first, usually by a high temperature thick film process. A layer of resistive material 14 provides resistive elements which are disposed over the glass 12 and substrate 10 and function as the heating elements that effect printing on the thermal medium. The glass 12 must be applied to facilitate optimal conduction of heat from resistive elements 14 to the substrate 10 such that enough heat is drawn off to allow proper cooling to optimize print speed while enough heat is retained for proper printing when an element is selected. The glass also serves to only slightly elevate functional elements for slightly better contact with a thermal sensitive medium. A layer of conductive material 16, typically aluminum or gold is deposited and patterned to form electrodes used to effect current flow to resistive material 14. Layers 18 and 20 are protective layers which serve to reduce head wear and resistor oxidation. In these prior art embodiments although resistive element characteristics may be controlled photolithographically, the glass bead 12 must be applied precisely and uniformly and composition of the glaze is a critical consideration. The composition of the glaze, which may include thermally conductive material, will depend on the dimensions of the glass bead. The glaze composition must also compensate for the thermal conductivity of the substrate 10. Depending on the substrate material, it may have thermal conduction properties that cause excess heat to be retained near the printing elements or excess heat to be conducted away therefrom.
While each of the various types of print heads can be found in use presently, it may be argued that true edge type thermal print heads enjoy advantages over center and near edge type heads. An edge type print head is illustrated in FIG. 3. Edge type typically have the resistive elements on an edge surface while conductive busses occupy a larger surface plane of the substrate. The substrate is usually orthogonally disposed with respect to the thermal paper which can be brought more uniformly into contact with the resistive elements disposed at the edge. The surface area of the edge can generally be shaped more evenly than top or bottom planes, resulting in higher quality printing because the resistive elements disposed thereon can be made more planar. Furthermore, with resistive elements disposed at an edge, less print head surface area comes into contact with the thermal recording paper, therefore: lesser pressing forces are required to maintain such contact; less wear occurs on the print head; the pressing mechanism may be simplified; and print quality is improved. Examples of edge type thermal print heads can be found in U.S. Pat. Nos. 4,399,348 and 4,636,811 to Bakewell and U.S. Pat. No. 4,651,168 to Terajima, et al.
Although edge type thermal print heads may be recognized as having advantages over center or near edge type thermal print heads, several problems have been identified with respect to this type of print head. Because edge type print heads are typically structures fabricated by alternately laminating conductive and insulating or dielectric layers on a substrate, as illustrated in the Bakewell patent and in FIGS. 2-10 of Terajima, dimensional considerations including physical and structural integrity of the substrate forming the base upon which resistive elements and conductive and insulating layers are disposed, may be critical. Considerable expense may arise from the need to assure that substrate surfaces are smooth and planar so that layers laminate properly thereon. The substrate or glaze layer disposed thereon must be uniformly dimensioned because resistor element length is determined by substrate width or glaze thickness and dot print uniformity affecting print quality is a function of the resistive element dimensions.
Although Terajima states that resistive element length may be controlled by the "simple expedient" of controlling film thickness of a glass layer applied to the substrate and that substrate smoothness may be effected by providing a glass layer between an electrode layer and the substrate, the provision of glass layers on the substrate is hardly a simple consideration. Since glaze thickness, according to the prior art, determines resistive element length and consequently resistive element values and because resistive element values determine print uniformity and quality, glaze thickness must be extremely precise and uniform. Furthermore, because the resistive elements come in contact with the glaze layer and the glaze layer effects thermal conductivity or thermal resistance, the glaze layer must be of a composition and amount such that resistive element thermal properties are optimized to facilitate proper printing on the thermal sensitive recording surface. The glaze layer must not be so thermally conductive that all the thermal energy is conducted away from the elements precluding printing. Likewise the glaze must not be so thermally resistant that all the thermal energy is concentrated at the elements without some conducted away. If thermal energy is retained at the elements print speed will be slowed because after heating up and printing each element must cool down so as not to print continuously. The element must properly heat up only when required to print and remain cool otherwise.
Additionally, application of glaze is generally a thick film process requiring high temperature deposition. Laminating high temperature dielectrics onto any thin metal layer (i.e. gold, aluminum, copper etc.) presents a problem of compromising the integrity of the metal layers, which likely will degrade when subject to the high temperatures required to fire the glaze.
In the embodiment of an edge type head shown in FIG. 3, a substrate 10' typically alumina, forms a base on which to deposit other layers. A first metallic layer is deposited and patterned to form a plurality of electrodes 22. A first dielectric or insulating layer 24 is deposited on top of the plurality of electrodes 22. A second metallic layer, deposited on the first insulating layer 24 is not patterned, but serves as a conducting ground plane 26. A second insulating layer or dielectric cover 28 protects ground plane 26. A plurality of resistive elements 30 functioning as the heating or writing elements which effect imaging on the thermally sensitive medium are connected to ground plane 26 and respective electrodes 22 along an edge of the laminated structure. This prior art embodiment requires several duplicative steps, such as separately depositing each metallic layer. It also requires that thick film insulating layers be deposited, normally at very high temperature, on the metallic layers which likely would be degraded by such temperatures.
Furthermore, this prior art embodiment requires precise dimensioning of insulative layer 24 because its thickness determines the length of resistive elements 30 which affect print quality as discussed hereinbefore.